The present invention relates to improvements in rotary drying drums of the type used in paper making machines, and particularly to an improved means of withdrawing the condensate from within the drum.
In a paper making machine, the web is formed and pressed and then passes to a dryer section including a series of dryer drums, each of which has a cylindrical shell with spaced heads and journals for rotatably mounting the shell. Conduits introduce high temperature steam into the shell and means are provided for withdrawing steam condensate from within the shell formed from the cooling of the steam as the traveling paper web is heated in passing over the drum for drying the web.
The steam within the drum must maintain the outer surface of the drum at a temperature for drying the paper web and transmit heat uniformly across the axial width of the drum so that the wide web will be uniformly dried. Water condensate forms within the shell as the steam gives off its heat to the shell. This condensate pools within the drum and at speeds of commercial operation, will tend to rim against the inner surface of the dryer shell forming an insulating barrier preventing heat transmission from the steam to the metal of the shell. To minimize the insulating effect of the layer of condensate rimming within the drum, removal devices have been provided to attempt to remove substantially all of the condensate. Conventionally, suction pipes are positioned within the shell having a removal opening close to the inner surface of the shell, but a layer of condensate will still remain. If the inlet end of the condensate removal tube is too close to the drum, interference will occur, and if is too far from the inner surface of the drum, the layer of condensate will be too thick and the insulating effect will be too great.
Efforts to reduce the insulating effect of the condensate within the drum and to break up the rimming effect have included positioning axially extending bars on the inner surface of the drum as illustrated by U.S. Pat. No. 3,217,426 and by U.S. Pat. No. 3,724,094. Difficulties have still been encountered in removal of the condensate and particularly in removing the condensate in such a manner that the insulating effect of the condensate which remains is uniform across the working face of the dryer drum. Where webs of paper are formed of widths of 16' and more, it is difficult to maintain the condensate at a uniform level for the axial working width of the drum, and if a greater amount of condensate remains at one axial location than another, the insulating effect will be uneven and, therefore, the surface temperature of the drum will be uneven causing uneven drying in the web. This is highly undesirable, and the web must depart from the drying section of the drum to be wound or calendered and be at a uniform moisture content across its entire width.
It is accordingly an object of the present invention to provide an improved dryer drum construction wherein the working face on the outer surface of the drum remains at uniform temperature over its axial length, and the insulating effect of the condensate within is maintained uniform for the length of the drum.
A further object of the invention is to provide an improved structure for removing condensate from within the dryer drum having axially extending bars for creating turbulence in the condensate within the drum, and which is particularly well suited for long drums as are necessary with paper machines which make wide width webs.